Field
This disclosure is generally related to solar cells. More specifically, this disclosure is related to a novel hole collector in a crystalline-Si (c-Si) based solar cell. The hole collector is formed by depositing a layer of high work function TCO and a layer of tunneling oxide on top of the c-Si base layer.
Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are many solar cell structures and a typical solar cell contains a p-n junction that includes a p-type doped layer and an n-type doped layer. In addition, there are other types of solar cells that are not based on p-n junctions. For example, a solar cell can be based on a metal-insulator-semiconductor (MIS) structure that includes an ultra-thin dielectric or insulating interfacial tunneling layer situated between a metal or a highly conductive layer and a doped semiconductor layer.
In a p-n junction based solar cell, the absorbed light generates carriers. These carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
To increase the conversion efficiency, a solar cell structure should allow the photon-generated carriers to effectively transport to the electrode. To do so, high quality carrier collectors for both types of carriers (electrons and holes) are needed. A typical p-n junction based solar cell includes a lightly n- or p-type doped base and a heavily doped emitter with an opposite doping type. For solar cells with an n-type doped emitter, electrons are collected by the n-type emitter, and the holes flow to the opposite side. The n-type doped emitter is also called an electron collector. To prevent minority carrier recombination at the surface the opposite side, a back surface field (BSF) layer (which is often a heavily doped layer having the same doping type as the base) can be formed at the surface of the opposite side. If the BSF layer is p-type doped, it collects holes. Similarly, for solar cells with a p-type doped emitter, holes are collected by the p-type emitter, and electrons flow to the opposite side to be collected by the n-type BSF layer.
Surface passivation is important for solar cell performance because it directly impacts the open circuit voltage (Voc). Note that a good Voc implies a good temperature coefficient, which enables a better solar cell performance at higher temperatures. One attempt to passivate the surface of the solar cell is to cover the surface of the Si absorber with materials having a wider bandgap, such as amorphous-Si (a-Si), or a thin layer of insulating material (such as silicon oxide or nitride). However, such passivation layers often impede current flows unintentionally. Such a current-impeding effect often has a greater impact on the holes than the electrons due to the fact that the valence band offset at the interface is larger than the conduction band offset. In addition, in Si, holes have much lower mobility than electrons. It is very difficult to get a high activation rate of p-type dopants. Hence, collecting hole current is often the bottleneck for further improving the fill factor.